The mounting of Integrated Circuits (“IC”) chips on Printed Circuit Boards (“PCBs”) requires inspection of the interconnections on the PCBs to determine whether the interconnections contain significant defects. Continual increases in the IC chip complexity, performance, and placement density place demands on the density and functionality of package interconnections. The Ball-Grid-Array (“BGA”) is one example of a Surface-Mount-Technology (“SMT”) package with interconnections that demand specialized inspection techniques. The continually increasing complexity and density of the PCB interconnections have resulted in the development of a number of interconnection inspection techniques for detecting defects on or within the interconnections.
One such interconnection inspection technique, tomosynthesis, is capable of detecting defects by creating a digital image representation of a sliced view along a single plane passing through a three-dimensional electrical solder joint connection. A digital tomosynthesis system makes it possible to inspect various PCB solder joint qualities, which cannot be inspected by visual methods or conventional X-ray radiography methods. U.S. Pat. No. 6,748,046 incorporated herein by reference, discloses a tomosynthesis inspection system. Also, U.S. Pat. No. 4,688,241 issued on Aug. 18, 1987 to Richard S. Peugeot, incorporated herein by reference, discloses a number of tomosynthesis inspection systems, including a system 10 depicted in FIG. 1 of the instant application. The system 10 includes a steerable microfocus X-ray source 12, a large-format image detector 30 capable of imaging X-rays, and an inspection plane 20 positioned between the source and the detector.
The regions A, B, and C to be imaged may be placed on an X-Y table (not shown), which lies in the inspection plane 20. When an object is on the X-Y table, the test object may be translationally moved along the x and y directions so that a region of interest, such as a solder joint, can be imaged. The source 12 produces an X-ray beam 50 having sufficient energy to penetrate the test object and reach the detector 30, while also having a low enough energy so that a resulting image has contrast within the region of interest.
The X-ray source 12 and the detector 30 may be mounted on independent vertical drive mechanisms allowing a continuously variable field-of-view, ranging from approximately 2.5 mm by 2.5 mm to approximately 25 mm by 25 mm, to be obtained. In particular, the X-ray source 12 is mounted on a programmable Z-axis, which changes the distance between the X-ray source 12 and the inspection plane 20. The distance between the X-ray source 12 and the plane 20 is referred to herein as Z1. The detector is also mounted on a programmable Z-axis, which changes the distance between the inspection plane 20 and the detector 30. The distance between the inspection plane 20 and the detector 30 is referred to herein as Z2. Variation of the field of view may be accomplished by varying either or both distances Z1 and Z2.
In operation, a circuit board having regions of interest A, B, and C is positioned on the X-Y table, in the inspection plane 20. The board is then moved translationally along the x and y directions so that a region of interest A, B, or C, such as a solder joint, or a component can be imaged. Once the board is properly positioned, a beam of radiation, such as X-ray beam 50, is projected towards an object on the circuit board. A portion of the X-ray beam 50 transmits through and is modulated by the object.
The portion of the beam 50 that passes through the object then strikes the image detector 30. The detector 30 is capable of producing an X-ray shadowgraph containing the modulation information from the test object. The X-rays striking the input screen of the detector 30 produce a visible light or shadowgraph image of the volume of the object that falls within the X-ray beam 50. If the detector 30 includes an image intensifier, the image at the output of the image intensifier is amplified in brightness.
The image that appears on the output face of the detector 30 is viewed, through a mirror, by a video camera (not shown). The images from various regions of the detector 30, such as the regions numbered 1, 3, 5 and 7 in FIG. 1, may be sequentially directed to the camera by adjusting the position of the mirror.
The resulting images are then input into a video digitizer. The video digitizer provides as an output digitized image sets. Each image in the set is supplied to a memory and stored. The images may then be separately fed into a tomosynthesis computer, which is programmed with a known tomosynthesis algorithm that effects a combination of the images and provides a resultant image to a monitor. In order to improve the resolution of the digitized image sets, it is desirable to limit the field of view of the camera to a region of the detector 30, such as the regions 1, 3, 5 or 7, rather than to acquire images for tomosynthesis viewing the entire detector 30.
For system 10, the center of the region of interest must coincide with a line extending from the center of the path of the x-ray source to the center of the detector 30. As can be seen in FIG. 1, the center of object B coincides with the centerline of X-ray beam 50 and the center of the field of view of detector 30.
To acquire tomosynthetic images for object B, for example, the X-ray source 12 is positioned at multiple points 1-8 along a circular path that is perpendicular to the Z axis. Each point on the circle falls in a plane that is perpendicular to the Z axis and maintains the same angle with, or is equidistant from, the Z axis. At each point, the X-ray source 12 emits an X-ray beam 50 towards, and at least partially through, the object B, thereby generating an image of object B at the detector 30. For example, to acquire image 1 for object B, the X-ray source 12 is steered to position 1 and the detector field of view is moved to position 1. This process is repeated for images 2 through 8 of object B. The 8 images are acquired sequentially since the electron beam inside the X-ray source housing and the detector field of view must be moved after each acquisition. As a result, 8 scanned images of object B at a known pre-determined angle are captured.
After the required images of object B are taken, then the X-Y table is moved so that the center of object A coincides with the centerline of the X-ray beam 50 and the center of the. detector field of view. To acquire image 1 for object A, the X-ray source 12 is steered to position 1 and the detector field of view is moved to position 1. This process is repeated for images 2 through 8 of object A. Thus, 8 scanned images of object A are captured. This process is continued for each of the objects, or regions of interest, to be imaged.
FIG. 2 shows a simplified illustration of a tomosynthetic image slice 200. As illustrated in FIG. 2, one problem encountered with some methods of tomosynthesis is that reconstruction artifacts 210 can appear in an image slice 200. The artifacts typically appear as white areas (or dark areas, if inverse image polarity) in the slice. In creating tomosynthetic images, the features on/in the circuit board such as ground planes, via holes, or other features, can create the reconstruction artifacts 210 in the tomosynthetic slices.
The artifacts 210 create several problems. First, the printed circuit boards are typically warped, so it is difficult to determine what level needs to be inspected. To find the level that needs to be inspected, the slices are searched across the expected point and to find out which is the best slice to inspect. An algorithm may be used to search the slices, for example the Zfind method disclosed in U.S. Pat. No. 7,013,038, by Rohit Patnaik, entitled METHOD FOR INSPECTING A BGA JOINT, issued Mar. 14, 2006, herein incorporated by reference in it entirety. The artifacts 210 contribute to the signal-to-noise ratio of the slice making it more difficult to isolate the best slice to examine. This can cause an isolation/search algorithm, such as Zfind, or other algorithm, to be unreliable. Thus, what is needed is a way to eliminate the artifacts.
Second, if there is noise in the final slice, a variety of measurement are made, typically about 30 different measurements for each solder connection. If there is noise in the final slice, it adds to the noise to the various measurements taken of the solder connections. With less noise, there could be more reliable results, with greater divergence of measurements results, to allow easier and clearer identification of good solder joints from bad ones. So, what is needed is a way to reduce artifacts to provide greater differentiation between good and bad solder connections.